Electric coupling circuit



Jan. 24, 1933. H. A. WHEELER ELECTRIC COUPLING CIRCUIT I Filed Aug. l9 1930.

ATTORNEYS Patented Jan. 24, 1933 UNETED STATES PATENT OFFICE HAROLD A. WHEELER, OF JACKSON HEIGHT S, NEW YORK, ASSIGNOB TO HAZELTINE CORPORATION, A CORPORATION OF DETJAWARE nnncrnxc oonrmne cmcurr' Application filed August 19, 1930. fierial Ito. 476,393.

This invention relates to high frequency electric coupling circuits tunable over a range in frequency and adapted more especially for interconnecting successive elements of a h gh frequency thermionic amplifier or radimsys- The coupling circuits of the present invention are characterized in that the voltage amplificat on produced therebv is caused to vary automatically with the frequency of tuning in a manner which is under the control of the designer; the design in general being preferably such as to produce a substantially con In-the elementary type of coupling circuit referred to there exists between the primary and tunable secondary circuits a degree of '3 coupling which is substantially constant throughout the tunable range, a condit on whichis partially responsible for the mentioned increase in amplification with frequen cy. The, present invention avoids the result- 5 ing d sadvantages by proper inclusion of suitable reactance elements adapted to provide an effective couplingbetween the input and tunable output circuitswhich automatically decreases with increase inthe tuned frequency and therebv may be proportionedto ofi'set .wholly or inpart the factors operative to .produce an increase in amplification with frequency.

Coupl'ng circuits broadly operative in ac 5 cordance with the principles disclosedherein but differing from the present invention as regards both circuit connections and thespecific modes of o eration thereof, are set forth in Patent No.1, 63,380 issued to C. E. Trube,

and in his copending applications Serial Nos.

101,906, filed A ril it, 1926 and 120,045 filed July 2, 1926.

The coupling circuits of the present inven- I tion embody the following essential elements, namely: a tunable secondary circuit including a fixed inductive winding and variable capac'ty bridged between output terminals, and a primary circuit including in closed series connection in the order named, a tapped portion of the secondary winding, a first primary coil uncoupled magnetically to said secondary w'nding, a fixed capacity and a second primary winding of relatively few turns compared to the said first prmary windin the second primary windingbeingmagnetica lycoupled to the secondary winding in such manner as to transfer energy from the primary tothe secondary circuit in additive rela-;

tion to that of the tapped portion of the secondary. In ut terminals are so connected to the close primary circuit as to provide therebetween a pair of parallel paths, one path containing the first primary winding and the tapped portion of the secondary winding while the other path contains the mg. l

' An additional capacity mayor ma not be connected between the input 'termina or in the operation of the circuit, capacity may exist between these terminals due to the type of input circuit connected thereto. Irrespective of the particular'arrangement used, the

1 resultant capacity associated with the inductance of the primary circuit, including the fixed capacity of the closed series connection as well as any capacity existing between the input terminals, should be such asto render the primary circuit resonant at'a heuuency e owest lower, but not greatly lower than th frequency within'the tunable range.

-The objects attained by such desi (1) provide an effective coupling between t e primary and secondarycircuits which automatically falls off as the resonance frequency of:

tuning is increased, but at a rate proportioned to produce a desiredvariation in amplification. with frequency, and (2) control the variations of the input impedance of the coupling circuit throughout the tunable range in sue fixed capacity and the second. primary windtively few turns compared to L ma manner as to insure improved stability, selectivity and amplification in the operation of the circuit.

The manner in which the results of the present invention are achieved will be more clearly understood upon detailed consideration of the circuit connections set forth in the drawini of which:

ig. 1 shows in diagrammatic form the elementary coupling circuit of this invention;

Fig. 2 show the coupling circuit of Fig. 1 as utilized for interconnecting anantenna circuit and the input to a thermionic tube;

Fig. 3 isa diagrammatic representation of the circuit as employed for interconnecting a pair of screen-grid thermionic tubes in cascade relation; while I Fig. 4 shows n interstage connection employing triode t ibes wherein the coupling circu'it is arranged in conjunction with a suitable capacity to provide neutralization of the interelectrode tube capacities.

Like elements are similarly designated in the several lfiguresf Referring to the drawing the coupling circuit includes a resonant secondary circuit consisting of a fixed inductive winding L and a variable tuning condenser C bridged between the output terminals 0. The primary circuit associated with input terminals I includes inclosed series connection in the order named; a portion L of secondary winding L, a fixed inductive winding L magnetically uncoupled from winding L, a fixed capacity C and a second primary winding L of relaly coupled to winding L as indicate The input terminals I are connected between the elements 0 and L and between L and L respectively, in order to provide the parallel paths therebetween one of which contains windings L and L while'the other path contains elements 0 and L The capacity Cgshown dotted, is intended to represent the entire capacity connected between the input terminals while the circuit is in operation, and in conjunction with capacity C together with any additional capacities associated with the primary circuit inductance which includes elements L L and L,, should be such as to tune the primary circuit to a frequency lower, but not greatly lower, than the lowest frequency within the tunable range as determined by adjustment of condenser C. It is not essential to the operation of the circuit that a capacity C, exist between input terminals I. In the event that i the capacity C, is zero, however, it is required that the capacity C be of such magnitude as to render the primary circuit resonant at the low frequency specified above.

The elements of the primary circuit as thus proportioned assure that the path between the input terminals I containing the elements C and L will be capacitively reeticalactive throughout the tunable range and hence of decreasing im dance for increases in tuned frequency; w ereas the path containing elements L and L, being inductively reactive, will increase in impedance with the frequency of tunin Thus the windings L and L produce e ects in the tuned circuit LC which vary in opposite sense with the tuning adjustment.

The circuit including the winding L has its greatest effect at the higher tuned frequencies due to approach to series resonance between condenser C and the relatively small effective inductance of winding L The circuit including the winding L, is most effective at the lower tuned frequencies due to approach to parallel resonance at the mentioned frequency below the tuning range of the primar circuit as a whole, i. e., windings L an L and the total capacity efi'ectively in shunt thereto.

By properly proportionin which vary oppositely with equency, and combining them in additive relation in the tuned secondary circuit, a resulting efiective coupling is obtained between primary and tuned secondary circuits WhlCll ma be caused automatically to decrease wit increase in tuned frequency, in a manner which is under control of the designer, and which with sufficient accuracy is expressed by the following equation": I

where f is the frequency of tuning, f the resonant frequency of the primary circuit and M and M being the mutual inductance 'bethese effects tween windings L and L and between L,

and L respectively, while the values of C and C are as indicated in the drawing.

The additive effects of windings L and L upon the tuned secondary circuit LC are obtamed by connecting windings L and L in opposite open circuit polarity to the lower input terminal -I. This reversal of connection is required to offset the substantial phase opposition of the prima flowing in the paths G and L,L,, respectively, resulting from the predominant .ca acitive reactance of the former and the in uctive reactance of the latter throughout the tunable frequency range.- I y In Fig. 2 the input terminals I of the coupling circuit are connected to an antenna 1 and a ground 2 respectively, while the output'terminals O are connectedto grid and cathode respectively of a thermionic tube V The capacity C of Fig. 1 in Fi 2 comprises the effective antenna-to-groun capacity of the antenna 1 which in conjunction with capacity C renders the primary circuit resocircuit currents pass condenser.

in-all of the Figures, 2 to 4 inclusive, the.

Space current for tube V is supplied through'a choke coil 6 connected directly from vthe anode thereof to the +13 tapof resistance 7 forming part of a direct current supply circuit. The blocking 'condenser 3 is interposed between the upper input terminal and the choke coil 6 to prevent grounding the anode potential through the coupling circuit.

The output terminals 0 of the coupling are connected respectively to the grid and through the bias circuit to the cathode of tube V The capacity C: of Fig. 1 comprises in Fig. 3 the anodc-toground capacity of tube V the capacity 3 in series therewith being sufficiently large that in general its effect may be neglected in comparison. The inductance of the primary circuit in conjunction with the associated capacities is of course resonant at a frequency slightly below the tunable range. a

The coupling circuit of'Fig. interconnects'a pair of triode tubes in cascade relation, each tube including anode, cathode and grid electrodes only. In order to neutralize the grid-to-anode capacity of tube V a condenser C is connected from the anode thereof to a point between the fixed capacity C and the primary winding L The neutralization is accomplished in the manner disclosed in Patent No. 1,450,080 to L. A. Hazeltine the magnetic coupling between windings L and L being close.

In Fig. 4 the total anode-to-ground capacity of tube V corresponds to the capacity C of Fig. 1. If tube V also has a neutralizing condenser like C connected to its anode, then this also is effectively in parallel with the input terminals. It will be noted that rotor element of the variable. condenser C is grounded, a very desirable feature slnce 1t prevents certain interstage couplings.

I claim:

1. An electrical coupling system compris-' ing a tunable secondary circuit including a fixed secondary winding and variable capacit'ybridged between output term nals, a primary circuit including in closed series connection, in the order recited. a portion of said secondary winding, a fixed inductance, a fixed capacity anda primary winding magnetically coupled to said secondary and of small inductance relative to said fixed inductance, a pair of input terminals connected to said primary circuit for providing shunt paths therebe-tween, one path including said fixed inductance and said secondary winding portion, and the other including said fixed capacity and primary winding, the inductance of said primary circuit in conjunction with associated capacity being resonant at a frequency below the tunable range for controlling the variation in coupling between said primary and'secondary circuits through-- out a tunable frequency range.

2. A high frequency electrical coupling circuit for interconnecting successive elements of a radio receiving system comprising in combination a tunable secondary circuit comprising fixed secondary inductance and variable capacity bridged between output terminals, a primary circuit including in closed series connection in the order recited a portion of said secondary inductance, a fixed inductance. a fixed capacity and a primary winding magnetically coupled to said secondary inductance and of small inductance relative to said primary inductance, and a pair of input terminals so connected to said primary circuit as to provide shunt paths therebetween, one path including said fixed pri' marv inductance and said secondary portion,

and the other path including said fixed capacity and primary winding, the inductance of said primary circuit in conjunction with the associated capacities, including any capacity effectively bridged between said input terminals, in the operation of said circuit being resonant at a frequency slightly below the tunable range for controlling the variation in coupling between said primary and secondary circuits throughout said frequency range.

3. In combination a high frequency electrical coupling circuit including a tunable secondary circuit comprising a fixed secondary winding and variable capacity bridged between a pair of output terminals, a primary circuit comprising in closed series connection in the order recited a portion of said secondary winding, 21 fixed inductance, a fixed capacity and a primary winding of small inductance relative to said fixed inductance and magnetically coupled to said secondary winding, a pair of input terminals connected between said fixed inductance and capacity and between said primary and said portion of said secondary windings respectively, and an input circuit connected to said input terminals, the inductance of said primary circuit, in'conjunction with said fixed capacity and any capacity effectively bridg ng said input terminals due to said input circuit, being resonant at a frequency slightly below the tunable range for controlling the variation in coupling between said primary and secondary circuits throughout said frequency range.

4. In combination a high frequency amplifier comprising an electrical coupling circuit connecting a pair of thermionic tubes in cascade relation each tube having anode, cathode 5 and control grid, said coupling circuit including a tunable secondary circuit comprising a fixed secondary winding and variable capacity bridged between grid and cathode of the second tube, and a primary circuit comprising in closed series connection a portion of said secondary winding, a fixedinductance, a fixed capacity and a primary winding of small inductance relative to said fixed inductance and magnetically coupled 5 to said secondary, a connection from the anode of said first tube to a point between said fixed inductance and capacity and from the cathode thereof to a point between said primary winding and said portion .of said secondary winding, the inductance of said primary circuit in conjunction with said fixed capacity and the anode-to-ground capacity of said first tube being resonant at a frequency slightly below the tunable range whereby in the operation of said amplifier uniformly high degrees of amplification are obtained throughout said frequency range. V

5. In combination a high frequency electrical coupling system connected between an antenna circuit and a thermionic tube having anode, cathode and control grid, said coupling'circuit comprising a fixed secondary winding and variable capacity bridged between said grid and cathode, and a primary circuit comprising in closed series connection, a portion of said secondary winding, a fixed inductance, a fixed capacity and a. primary winding magnetically coupled to said secondary,-'said antenna circuit being joined to input terminals connected between said fixed inductance and capacity and between said primary winding and said portion of said secondary winding respectively, the inductance of said primary circuit in conjunction with said fixed capacity and the capacity of said antenna circuit effectively between said input terminals being resonant at a frequency slightly below the tunable range whereby in the operation of said systemunitained throughout said frequency range.

6. A high frequency electrical coupling circuit for interconnecting successive elementsof a multi-stage thermionic radio receiver or amplifier comprisinga tunable secondary circuit including a fixed secondary winding and variable capacity bridged between a pair of output terminals, and a primary circuit comprising a portion of said sec- C3 ondary winding and a fixed primary inductance serially connected between a pair of input terminals, together with a fixed capacity and a primary winding of small inductance relative to said primary inductance, also serially connected between formly high degrees of amplification are obsaid input terminals, the inductance of said prlmary circuit in conjunction with all associated capacities in the operation of said circuit being resonant at a frequency below the tunable range, said primary winding being coupled magnetically to said secondary winding and connected with opposite open circuit polarity relative to that of said secondary winding.

7. In combination a pair of thermionic tubes each including anode, cathode and control grid, and a high frequency coupling circuit connecting said tubes in cascade relation, said circuit including a fixed secondary winding and'variable capacity in parallel joined at one side to the grid-of the second tube and at the opposite side to said cathodes in common, a tap from an intermediate point of said secondary winding through fixed inductance to the anode of the first tube, a primary winding of small inductance relative to said fixed inductance and magnetically coupled to said secondary, said primary winding being connected from said cathode of said first tube through a fixed capacity to the anode thereof, and a condenser proportioned to neutralize the grid-to-anode capacity of said second tube connected-from the anode thereof to a point between said fixed capacity and said primary winding, said primary winding having opposite polarity relative to that of said secondary winding.

8. In combination a high frequency electrical circuit coupling a pair of thermionic tubes in cascade relation, each tube including anode, cathode and grid, said circuit comprising a tunable secondary circuit including a fixed inductive winding shunted by variable capacity bridged from the grid of said second tube to said cathodes in common, a tap from an intermediate point of said secondary winding thru a fixed inductance to the anode of said first tube, a primary winding of small magnitude relative to said fixed inductance connected at one terminal to said cathodes and at its opposite terminal through a fixed capacity to the anode of said first tube, andto the anode of said second tube through a capacity proportioned to neutralize the anode-to-grid capacity thereof, said primary winding being so coupled magnetically to'said secondary as to accomplish said neutralization, and said fixed inductance of said coupling circuit in conjunction with the associated capacities including the anode-toground capacity of said first tube, being resonant at a frequency slightly below the tunable range whereby in the operation of said circuit uniformly high degrees of amplification are obtained throughout said range.

In testimony whereof I afiix my signature.

HAROLD A. IVHEELER. 

